Permanent antistatic primer layer

ABSTRACT

There is provided a method for preparing a biaxially oriented polyester sheet or web, with improved antistatic properties, comprising the steps of 
     (i) stretching said polyester sheet or web first in one direction and second in a direction perpendicular thereto 
     (ii) coating said hydrophobic polyester sheet or web, either before stretching or between said first and second stretching operation, on one or both sides, with a transparent antistatic primer layer, wherein the coating composition of said transparent antistatic primer layer comprises (1) a dispersion of a polythiophene with conjugated polymer backbone and a polymeric polyanion compound and (2) a latex polymer having hydrophilic functionality. 
     The primer layer is coated from an aqueous composition and does not show a substantial change in resistivity depending on relative humidity or wet processing.

FIELD OF THE INVENTION

The present invention relates to a sheet or web material havingantistatic properties, and is particularly but not exclusively concernedwith recording materials wherein a hydrophobic resin support carries anantistatic subbing layer imparting good adherence to a hydrophiliccolloid-containing layer, e.g. a light-sensitive gelatin-silver halideemulsion layer.

BACKGROUND OF THE INVENTION

It is known that hydrophobic resin sheet and web materials of lowconductivity readily become electrostatically charged by friction withdielectric materials and/or contact with electrostatically chargeabletransport means, e.g. rollers. The charging occurs particularly easilyin a relatively dry atmospheric environment.

Sheets and webs of hydrophobic resins, e.g. polyesters orcellulosetriacetate, are commonly used as support element of recordingmaterials. Such materials are subjected to frictional contact with otherelements during their manufacture, e.g. during coating or cutting, andduring use, e.g. during the recording of information, e.g. with astep-and-repeat camera or in the case of silver halide photographicmaterials for X-ray diagnosis during use in filmchangers or when used infilm loading and unloading in so-called daylight systems.

Especially in the reeling-up or unreeling of dry photographic film in acamera or projector high friction may occur, resulting in electrostaticcharges that may attract dust or cause sparking. In unprocessedphotographic silver halide emulsion materials sparking gives rise todevelopable fog and degrades the image quality.

In order to reduce electrostatic charging of photographic sheet or webmaterials comprising a hydrophobic resin support coated with at leastone silver halide emulsion layer without impairing their transparency itis known to incorporate ionic compounds in these materials, e.g. in thegelatin-silver halide emulsion layer(s) or other hydrophilic colloidlayers or in a subbing layer.

In order to avoid diffusion of ionic compounds out of said layers duringthe wet processing treatments of said materials, preference has beengiven to incorporate therein antistatic high molecular weight polymericcompounds having ionic groups, e.g. carboxylic sodium salt groups, atfrequent intervals in the polymer chain [ref. Photographic EmulsionChemistry, by G. F. Duffin,--The Focal Press--London and New York(1966)--Focal Press Ltd., p. 168]. To further enhance the permanence ofthe conductivity of ionic conductive polymers it has been proposed tocross-link these polymers with hydrophobic polymers (ref. e.g. U.S. Pat.Nos. 4,585,730, 4,701,403, 4,589, 570, 5,045,441, EP-A-391 402 andEP-A-420 226).

The conductivity however of an antistatic layer containing said ionicconductive polymers, even after cross-linking, is moisture dependent andis lowered considerably by treatment with an acidic photographicprocessing liquid, e.g. an acidic photographic fixing liquid or stopbath.

Relatively recently electrically-conducting conjugated polymers havebeen developed that have electronic conductivity. Representatives ofsuch polymers are described in the periodical Materials & Design Vol.11, No. 3--June 1990, p. 142-152, and in the book "Science andApplications of Conducting Polymers"--Papers from the 6th EuropeanPhysical Society Industrial Workshop held in Lothus, Norway, 28-31 May1990, Edited by W R Salaneck Linkoping University, D T Clark ICI WiltonMaterials Research Centre, and E J Samuelson University of Trondheim,published under the Adam Hilger imprint by IOP Publishing Ltd TechnoHouse, Redcliffe Way, Bristol BS16NX, England.

Substances having electronic conductivity instead of ionic conductivityhave a conductivity independent from moisture. They are particularlysuited for use in the production of antistatic layers with permanent andreproducible conductivity.

Many of the known electronically conductive polymers are highly colouredwhich makes them less suited for use in photographic materials, but someof them of the group of the polyarenemethylidenes, e.g. polythiophenesand polyisothianaphthene are not prohibitively coloured and transparent,at least when coated in thin layers.

The production of conductive polythiophenes is described in preparationliterature mentioned in the above mentioned book: "Science andApplications of Conducting Polymers", p. 92.

The production of colour neutral conducting polymers fromisothionaphthene is described in J Electrochem Soc 134, (1987) 46.

For ecological reasons the coating of antistatic layers should proceedwhere possible from aqueous solutions by using as few as possibleorganic solvents. The production of antistatic coatings from aqueouscoating compositions being dispersions of polythiophenes in the presenceof polyanions is described in published European patent application 0440 957 and corresponding U.S. Ser. No. 647,093 which should be read inconjunction herewith.

It is known that the electrostatic chargeability of polyester resin ishigh and that for many applications it would be interesting to lower itssurface resistance e.g. by applying thereto a conductive primer orsubbing layer. It is however also known that it is difficult toestablish a good bonding between a polyester resin support and ahydrophilic antistatic layer. In most cases more than one layer isneeded to impart sufficient adherence of a hydrohilic colloid layer to apolyester resin support as is the case e.g. in photographic materials,having one or more hydrophilic colloid recording layers such asgelatin-silver halide emulsion layers. Normally, a first specialadhesion layer is coated on the support to adhere thereto a properantistatic layer, which may be protected with a protective layer foravoiding mechanical damage and attack by solvents.

It is common practice to give a polyester film support a sufficientdimensional stability by biaxially stretching and to heat-set it atrelatively high temperature. Usually the biaxially orientation of thepolyester film support is performed in two stages. First the film isstretched in one direction and afterwards in a direction perpendicularto the first. From an economic point of view it would be advantageous ifan antistatic primer layer on the polyester film could be applied eitherbefore or between the stretching operations. Applied in the stretchingstage the primer layer should retain a good anchorage and its elasticmodulus should be such that it easily follows the film enlargement inthe stretching. The elastic modulus is the ratio of stress (force perunit area) to strain, the latter being a pure number representing thepercentage of elongation (ref. Sears & Zemansky "University Physics",4th ed.--Addison-Wesley Publishing Company--Reading, Mass., USA, p.154-155).

After the biaxial stretching the film is conducted through aheat-setting zone wherein the primed polyester film such as apolyethylene terephthalate film is heated until a temperature between180° and 220° C. is reached, while the film is kept under tension inboth directions.

The primer layer should withstand these temperatures without.prohibitive colouration and not loose its conductivity whenincorporating electrically conductive material.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a sheet or webmaterial which includes a hydrophobic polyester resin support, carryinga transparent antistatic primer layer that possesses good adherance tosaid support in dry and wet state and can be subjected to stretchingtogether with said polyester resin support without loosing its adherancethereto and without substantial colouration in the heat-treatment duringand following said stretching.

It is a further object of the present invention to provide a sheet orweb type photographic gelatin-silver halide emulsion layer materialcontaining said primed support, that through said antistatic primerlayer obtains permanent antistatic properties, i.e. does not show asubstantial change in resistivity depending on relative humidity of theatmosphere or the influence of wet processing.

Other objects and advantages of the present invention will become clearfrom the further description and examples.

In accordance with the present invention there is provided a method forpreparing a biaxially oriented polyester sheet or web, with improvedantistatic properties, comprising the steps of

(i) stretching said polyester sheet or web first in one direction andsecond in a direction perpendicular thereto

(ii) coating said hydrophobic polyester sheet or web, either beforestretching or between said first and second stretching operation, on oneor both sides, with a transparent antistatic primer layer, wherein thecoating composition of said transparent antistatic primer layercomprises (1) a dispersion of a polythiophene with conjugated polymerbackbone and a polymeric polyanion compound and (2) a latex polymerhaving hydrophilic functionality.

By "latex polymer" is understood a polymer or copolymer that is appliedas an aqueous dispersion (latex) of particles of said polymer orcopolymer.

By "hydrophilic functionality" is meant a chemical group having affinityfor water e.g. a sulphonic acid or carboxylic acid group preferably insalt form e.g. an alkali metal salt group.

DETAILED DESCRIPTION OF THE INVENTION Polythiophene/polyanion dispersion

Preferably said polythiophene has thiophene nuclei substituted with atleast one alkoxy group, or --O(CH₂ CH₂ O)_(n) CH₃ group, n being 1 to 4,or thiophene nuclei that are ring closed over two oxygen atoms with analkylene group including such group in substituted form. "Ring closedover two oxygen atoms" has to be understood that two adjacent carbonatoms of the thiophene ring together with an oxy-alkylene-oxy group forman adjacent ring. Preferred polythiophenes for use according to thepresent invention are made up of structural units corresponding to thefollowing general formula (I): ##STR1## in which: each of R¹ and R²independently represents hydrogen or a C₁₋₄ alkyl group or togetherrepresent an optionally substituted C₁₋₄ alkylene group or acycloalkylene group, preferably an ethylene group, an optionallyalkyl-substituted methylene group, an optionally C₁₋₁₂ alkyl- orphenyl-substituted 1,2-ethylene group, a 1,3-propylene group or a1,2-cyclohexylene group.

The preparation of said polythiophene and of aqueouspolythiophene-polymeric polyanion dispersions containing saidpolythiophene is described in published European patent application 0440 957 and corresponding U.S. Ser. No. 647,093.

The preparation of said polythiophene proceeds in the presence of saidpolymeric polyanion compounds by oxidative polymerization of3,4-dialkoxythiophenes or 3,4-alkylenedioxythiophenes according to thefollowing general formula (II): ##STR2## wherein: R¹ and R² are asdefined in general formula (I), with oxidizing agents used e.g. for theoxidative polymerization of pyrrole and/or with oxygen or air in thepresence of said polyacids, preferably in aqueous medium containingoptionally a certain amount of organic solvents, at temperatures of 0°to 100° C.

The polythiophenes get positive charges by the oxidative polymerization,the location and number of said charges is not determinable withcertainty and therefore they are not mentioned in the general formula ofthe repeating units of the polythiophene polymer.

When using air or oxygen as the oxidizing agent their introductionproceeds into a solution containing thiophene, polyacid, and optionallycatalytic quantities of metal salts till the polymerization is complete.

Oxidizing agents suitable for the oxidative polymerization of thiophene,according to the present invention, may be the same as those that aredescribed, for example, in J. Am. Soc. 85, 454 (1963) as suitable forthe oxidative polymerization of pyrrole. Inexpensive and easy-to-handleoxidizing agents are preferred such as iron(III) salts, e.g. FeCl₃,Fe(ClO₄)₃ and the iron(III) salts of organic acids and inorganic acidscontaining organic residues, likewise H₂ O₂, K₂ Cr₂ O₇, alkali orammonium persulfates, alkali perborates, potassium permanganate andcopper salts such as copper tetrafluoroborate.

Theoretically, 2.25 equivalents of oxidizing agent per mol of thiopheneare required for the oxidative polymerization thereof [ref. J. Polym.Sci. Part A, Polymer Chemistry, Vol. 26, p.1287 (1988)]. In practice,however, the oxidizing agent is used in a certain excess, for example,in excess of 0.1 to 2 equivalents per mol of thiophene.

For the polymerization, the thiophenes corresponding to the abovegeneral formula (II) a polyacid and oxidizing agent are dissolved oremulsified in an organic solvent or preferably in water and theresulting solution or emulsion is stirred at the envisagedpolymerization temperature until the polymerization reaction iscompleted. By that technique stable aqueous polythiophene dispersionsare obtained having a solids content of 0.05 to 55% by weight andpreferably of 0.1 to 10% by weight.

The polymerization time may be between a few minutes and 30 hours,depending on the size of the batch, the polymerization temperature andthe kind of oxidizing agent. The stability of the obtainedpolythiophene/polyanion dispersion may further be improved during and/orafter the polymerization by the addition of dispersing agents, e.g.anionic surface active agents such as dodecyl sulfonate, alkylarylpolyether sulfonates described in U.S. Pat. No. 3,525,621.

The size of the polythiophene particles in the polythiophene/polyaniondispersion is in the range of from 5 nm to 1 μm, preferably in the rangeof 40 to 400 nm.

Suitable polymeric polyanion compounds for use in the presence of saidpolythiophenes, according to the present invention, are provided byacidic polymers in free acid form or by a salt of said acidic polymers.The acidic polymers are preferably polymeric carboxylic or sulphonicacids. Examples of such polymeric acids are polymers containingrepeating units selected from the group consisting of acrylic acid,methacrylic acid, maleic acid, vinyl sulfonic acid and styrene sulfonicacid or mixtures thereof.

The anionic (acidic) polymers used in conjunction with the dispersedpolythiophene polymer to form the polythiophene/polyanion dispersion tobe used according to the present invention, have preferably a content ofanionic groups of more than 2% by weight with respect to said polymercompounds to ensure sufficient stability of the dispersion. Suitableacidic polymers or corresponding salts are described e.g. in DE-A -25 41230, DE-A-25 41 274, DE-A-28 35 856, EP-A-14 921, EP-A-69 671, EP-A-130115, U.S. Pat. Nos. 4,147,550, 4,388,403 and 5,006,451.

The polymeric polyanion compounds may consist of straight-chain,branched chain or crosslinked polymers. Cross-linked polymeric polyanioncompounds with a high amount of acidic groups are swellable in water andare named microgels. Such microgels are disclosed e.g. in U.S. Pat. Nos.4,301,240, 4,677,050 and 4,147,550.

The molecular weight of the polymeric polyanion compounds beingpolyacids is preferably in the range from 1,000 to 2,000,000 and morepreferably in the range from 2,000 to 500,000. Polyacids within theabove criteria are commercially available, for example polystyrenesulfonic acids and polyacrylic acids, or may be produced by knownmethods (ref. e.g. Houben-Weyl, Methoden der Organischen Chemie, Vol.E20, Makromolekulare Stoffe, Teil 2, (1987), pp. 141 et seq.).

Instead of the free polymeric polyacids applied in conjunction with thepolythiophenes it is possible to use mixtures of alkali salts of saidpolyacids and non-neutralized polyacids, optionally in the presence ofmonoacids. Free acid groups of the polyanionic polymer may be allowed toreact with an inorganic base, e.g. with sodium hydroxide, to obtain aneutral polymer dispersion before coating.

The weight ratio of polythiophene polymer to polymeric polyanioncompound(s) can vary widely, for example from about 50/50 to 15/85.

The Latex Polymer

The latex polymer used in conjunction with the said polythiopheneprovides through its film forming character upon drying and itshydrophilic functionality a good bonding between the polyester filmsupport and thereon applied hydrophilic layers.

The "latex polymer" applied in admixture with said polythiophene andpolymeric anion compound is preferably a copolyester containingsulphonic acid groups in salt form, but other polyesters, such as thecopolyesters having hydrophilic functionality as described e.g. in U.S.Pat. Nos. 3,563,942, 4,252,885, 4,340,519, 4,394,442 and 4,478,907, maybe used likewise.

Preferred copolyesters contain a certain amount of sulphonic acid groupsin salt form (ref. GB-P 1,589,926) and as described in U.S. Pat. No.4,478,907 and EP 78 559 and for raising their glass transitiontemperature (Tg) contain an amount of particular co-condensatedcross-linking agent(s). Such copolyesters contain e.g. recurring estergroups derived from ethylene glycol and an acid mixture containing (i)terephthalic acid, (ii) isophthalic acid, (iii) 5-sulphoisophthalic acidwhose sulpho group is in salt form and (iv) a polyfuctional acidproducing crosslinks.

In a particularly preferred embodiment the copolyester is a copolyestercontaining recurring ester groups derived from ethylene glycol and anacid mixture containing terephthalic acid, isophthalic acid and5-sulphoisophthalic acid whose sulpho group is in salt form, said acidmixture consisting essentially of from 20 to 60 mole % of isophthalicacid, 6 to 10 mole % of said sulphoisophthalic acid, 0.05 to 1 mole % ofcross-linking agent being an aromatic polycarboxylic acid compoundhaving at least three carboxylic acid groups or corresponding acidgenerating anhydride or ester groups, the remainder in said acid mixturebeing terephthalic acid.

Although said copolyesters, having hydrophilic functionality throughsaid sulfonic acid (salt) group are preferred, likewise addition(co)polymers having hydrophilic functionality may be used withsatisfactory results.

According to a particular embodiment of the present invention thecoating composition of the primer layer contains in admixture to thealready mentioned polythiophene/polyanion dispersion and copolyesterlatex, an addition polymer or copolymer in latex form and havinghydrophilic functionality. Such (co)polymers are disclosed in e.g.EP-A-386 529, U.S. Pat. No. 3,861,924, Research Disclosure No. 162,October 1977 p. 47-49 item 16 258, U.S. Pat. Nos. 4,225,665 and4,689,359 . Further polyurethanes having hydrophilic properties asdisclosed in U.S. Pat. No. 4,388,403 may be used as latex polymers.

In the coating composition for coating an antistatic primer layeraccording to the present invention, said addition polymer is preferablya copolymer formed from 45 to 70% by weight of a lower (C₁ -C₄) alkylmethacrylate, 25 to 50% by weight of butadiene and from 2 to 5% byweight of an ethylenically unsaturated carboxylic acid.

An example of a very suitable addition polymer applied in latex form andhaving hydrophilic functionality is a copolymer of vinylidene chlorideand an unsaturated carboxylic acid as described e.g. in U.S. Pat. No.3,649,336. Preferably such an addition copolymer is a copolymer formedfrom 60 to 90% by weight of vinylidenechloride, 9 to 30% of a lower (C₁-C₄) alkyl methacrylate and from 1 to 5% by weight of an ethylenicallyunsaturated carboxylic acid.

Coating of the Antistatic Primer Layer

The anstistatic layer according to the present invention can be coatedonto the polyester sheet or web before the stretching operations of saidpolyester sheet or web material. Advantageously the coating proceedsbetween the two stretching operations of a polyethylene terephthalatefilm web, wherein the stretching ratio of both stretching operation maybe between 2.5:1 to 4.0:1.

The anstistatic .layer of this invention may have after drying andco-stretching with the resin support a thickness in the range of 0.001to 5 μm, preferably the thickness is in the range of 0.01 to 0.μpm.

The anstistatic layer according to this invention can be coated on oneor on both sides of the support and to improve the spreading of theantistatic primer layer a surface active agent may be added to thecoating solution. For that purpose anionic as well as non-ionic wettingagents may be used, e.g. a C₁₂ -C₁₈ alkylphenol polyethyleneoxide ethersuch as dodecylphenol polyethylene oxide ether, p-nonyl phenoxypolyglycidol, iso-octyl-phenylene-(O--CH₂ --CH₂)₈ --O--CH₂ --COOH soldunder the tradename AKYPO OP80 by CHEMY, the Netherlands, or saponine.It is clear that other surface-active agents may be used and that theirfavourable influence on reduction of surface resistivity can be checkedby simple tests. A survey of surface-active agents is given e.g. inTensid-Taschenbuch Herausgegeben yon Dr. Helmut Stache--Carl HanserVerlag Munchen Wien (1979).

The coating composition may contain aliphatic polyhydroxy compounds asstretch improving agents (e.g. glycerol, sorbitol, etc.) as described inU.S. Pat. No. 4,089,997 and a monomeric carboxylic acid as a viscosityregulating agent (e.g. furancarboxylic acid, malonic acid and glutaricacid) as described in U.S. Pat. No. 4,089,997.

The coating composition may contain additional antistatic agents e.g.metal oxides as described e.g. in U.S. Pat. No. 5,006,451 or colloidalsilica having preferably an average particle size not larger than 100nm, preferably not larger than 50 nm.

Colloidal silica having an average grain size between 5 and 100 nm isavailable in aqueous colloidal dispersions and is marketed under thecommercial names LUDOX (trade name of E. I. du Pont de Nemours,Wilmington, Del. U.S.A., and SYTO (trade name of Monsanto ChemicalCorporation, Boston Mass. USA) and KIESELSOL (trade name of Bayer AG,Leverkusen, Germany). Particularly suited is KIESELSOL 100 F wherein thecolloidal silica has a particle size in the range of 25 to 30 nm.

In the preparation of a photographic silver halide emulsion layermaterial the anstistatic primer layer can be coated directly with thesilver halide emulsion.

When a single sided silver halide emulsion layer material is prepared ona polyester sheet or web material that is coated on both sides with anantistatic primer layer according to the present invention, the sideopposite to the silver halide emulsion layer can have on the anstistaticlayer a protective layer consisting of an hydrophobic organic polymerhaving a glass transition temperature value (Tg) of at least 40° C. tobecome an outermost backing layer. The hydrophobic polymer used asprotective layer on a anstistatic layer according to this invention ispreferably polymethylmethacrylaat having a Tg value above 100° C.

The antistatic primer layer of the present invention can be coated witha hydrophilic subbing layer containing a certain amount of hydrophiliccolloid such as gelatin.

Such subbing layer may have a composition as described for layer (B) inU.S. Pat. No. 3,649,336 and corresponding GB-P 1,234,755. Said subbinglayer comprises e.g. in a weight ratio of 1:3 to 1:0.5 a mixture ofgelatin and colloidal silica. The coating composition of said subbinglayer may contain wetting agents improving the spreading of the coatingand plasticizers, e.g. polyoxyalkylene compounds and polyols and thegelatin plasticizers described in published EP-A 0 078 559 improving theflexibility of the coating.

Further said subbing layer may contain matting agents, biocides,light-absorbing or reflecting pigments, e.g. carbon black and/ortitanium dioxide and/or bleachable dyes.

The thickness of the dried subbing layer may vary between 0.10 and 5 μm,and is normally about 1 μm.

A photographic silver halide emulsion material containing an antistaticprimer layer according to the present invention as defined hereinbeforemay contain (a) silver halide emulsion layer(s) of any type known tothose skilled in the art. For example, these materials may contain asilver halide emulsion layer of the type used in continuous tone orhalftone photography, microphotography and radiography. The definedantistatic primer layer be used advantageously in black-and-white orcolour photographic materials and likewise in silver halide emulsionlayers intended for use in the silver complex diffusion transferreversal (DTR) process as well as in a dye diffusion transfer processoperating with silver halide emulsion layers.

For the composition of silver halide emulsion layers reference is madee.g. to Research Disclosure 17,643 of December 1978 and ResearchDisclosure 307,105 of November 1989.

Apart from light-sensitive hydrophilic gelatin-silver halide emulsionlayers any other gelatin-containing layer that has not to bephotosensitive may be applied directly to the anstistatic primer layeror to the subbed anstistatic layer. For example, said gelatin-containinglayer is an anti-halation layer, anti-curl layer, gelatinousimage-receiving layer containing physical development nuclei for use indiffusion transfer reversal photography (DTR-photography) or is agelatinous image-receiving layer containing a mordant for image-wisetransferred dyes. The principles and embodiments of silver imageformation by DTR-photography are described e.g. by Andre Rott and EdithWeyde in the book "Photographic Silver Halide Diffusion Processes"--TheFocal Press London and New York (1972), and the principles andembodiments of the production of colour images by dye diffusion transferare described e.g. by C. Van de Sande in Angew. Chem. Int. Ed. Engl. 22,(1983) p. 191-209.

The coating of a hydrophilic gelatin-containing subbing layer on theanstistatic primer layer may be omitted when the layer to be adhered tosaid antistatic primer layer does not contain gelatin but includes aless hydrophilic colloidal binder material, e.g. polyvinyl alcoholderived from partially hydrolyzed polyvinyl acetate, and alkali-solubleacrylic type polymers and copolymers containing free carboxylic acidgroups, which polymers may be applied in formulations for the productionof photo-resists as described e.g. in European Patent Specification (EP)0 036 221.

By the above defined antistatic primer layer it is possible to reducethe surface resistance of a resin sheet or web material to a value lowerthan 10¹⁰ Ω/square at relative humidity (R. H.) of 30%.

The surface resistance expressed in Ω/square (ohm/sq.) of the abovedefined antistatic layer is measured according to test procedure A asfollows:

after coating, the resulting antistatic layer or its assemblage withoverlying layer(s) is dried and conditioned at a specific relativehumidity (R.H.) and temperature. The surface resistance expressed in ohmper square (Ω/square) is performed by placing onto the outermost layertwo conductive poles having a length of 10 cm parallel to each other ata distance of 1 cm and measuring the resistance built up between theelectrodes with a precision ohm-meter (ref. DIN 53482).

According to test procedure B (described in the periodical ResearchDisclosure--June 1992, item 33840) the resistance of the layerassemblage is measured contact-less by arranging it between capacitorplates making part of a RC-circuit differentiator network. Thedimensions of the measurement cell are chosen in such a way that relyingon the known capacitor value (C) it is possible to calculate from themeasured RC-value the electrical resistance of the layer assemblage.Such proceeds by introducing an electrical pulse into the measurementcircuit and recording the discharge curve which yields the time τ=R×C,wherein the applied charge and voltage of the electrical pulse havedroppped to its 1/e value (e is the base number of the naturallogarithms). Applying an alternating current voltage with frequency (f),considering the RC-circuit as a high frequency pass filter, it ispossible to find the resistance by using the equation: f=1/2π×R×C at the3 dB point.

The discharge duration (termed as τ) to the value 1/e of the originalvoltage and charge in the series resistance (R) and capacitor (C)circuit is expressed in milliseconds (msec). The lower that value thebetter the antistatic character or charge mobility of the appliedantistatic layer.

The present invention is illustrated by the following examples without,however, limiting it thereto. The ratios and percentages are by weightunless otherwise indicated.

In the examples, described hereafter, following preparation were used:

PREPARATIONS I to IV (I) Preparation of 3,4-ethylenedioxy-thiophene

The 3,4-disubstituted thiophenes of the above general formula (II) canbe obtained by processes known in principle by reacting the alkali metalsalts of 3,4-dihydroxy-thiophene-2,5-dicarboxylic esters with theappropriate alkylene vic-dihalides and subsequently decarboxylating thefree 3,4-(alkylene-vic-dioxy)thiophene-2,5-dicarboxylic acids (see, forexample, Tetrahedron (1967) Vol. 23, 2437-2441 and J. Am. Chem. Soc. 67(1945) 2217-2218).

(II a) Preparation of poly(3,4-ethylenedioxy-thiophene)/polyaniondispersion called hereinafter dispersion (PT1)

a) Into 1000 ml of an aqueous solution of 7 g of polystyrene sulfonicacid (109 mmol of SO₃ H groups) with number-average molecular weight(Mn) 40,000, were introduced 12.9 g of potassium peroxidisulfate (K₂ S₂O₈), 0.1 g of Fe₂ (SO₄)₃ and 2.8 g of 3,4-ethylenedioxy-thiophene. Thethus obtained reaction mixture was stirred for 24 h at 20° C. andsubjected to desalting.

b) The above prepared reaction mixture was stirred for 6 hours at roomtemperature in the presence of a granulated weak basic ion exchangeresin LEWATIT H 600 (tradename of the Bayer Company of Leverkusen,Gemany) and strongly acidic ion exchanger LEWATIT S 100 (tradename ofthe Bayer Company of Leverkusen, Germany). After said treatment the ionexchange resins were filtered off and the potassium ion and sulfate ioncontent were measured which were respectively 0.4 g K⁺ and <0.1 g(SO₄)²⁻ per liter.

(II b) Preparation of poly(3,4-ethylenedioxythiophene)/polyaniondispersion called hereinafter dispersion (PT2)

a) Into 1000 ml of an aqueous solution of 14 g of polystyrene sulfonicacid (218 mmol of SO₃ H groups) with number-average molecular weight(Mn) 40,000, were introduced 12.9 g of potassium peroxidisulfate (K₂ S₂O₈), 0.1 g of Fe₂ (SO₄)₃ and 5.68 g of 3,4-ethylenedioxy-thiophene. Thethus obtained reaction mixture was stirred for 24 h at 20° C. andsubjected to desalting.

b) 500 ml of the above prepared reaction mixture were diluted with 500ml of water and stirred for 6 hours at room temperature in the presenceof a granulated weak basic ion exchange resin LEWATIT H 600 (tradenameof Bayer AG, Leverkusen, Gemany) and strongly acidic ion exchangerLEWATIT S 100 (tradename of Bayer AG, Leverkusen, Germany). After saidtreatment the ion exchange resins were filtered off and the potassiumion and sulfate ion content were measured which were respectively 0.4 gK⁺ and 0.1 g (SO₄)²⁻ per liter.

(III) Preparation of the copolyester latex P (III a) Preparation ofcompound CA

In a 1-liter round-bottomed flask with a stirrer, a reflux-condenser, athermometer and dropping funnel were introduced 41.8 g (0.2 mole) of5-amino-isophthalic acid dimethyl ester, 18.48 g (0.22 mole) of sodiumhydrogen carbonate and 400 ml of anhydrous acetone. The mixture isheated under reflux for a short time and then cooled down again to roomtemperature. At that temperature 15.5 g (0.1 mole) of succinylchloridedissolved in 25 ml of anhydrous acetone were added dropwise whilststirring. The reaction product precipitated during that operation. Thewhole mixture was then kept stirring for 1 h at room temperature and for1 h at reflux temperature. Thereupon the reaction mixture was cooled inice and the precipitated reaction product separated by suction filteringand dried in vacuum.

The crude product can be purified by washing with water andrecrystallization from dimethylformamide.

In the polycondensation reaction for forming the copolyester the abovepolyfunctional carboxylic acids increase the Tg-value of thecopolyester. Properly chosen proportions of sulpho groups, the degree ofpolycondensation and cross-linking provide the required dispersibilityin water of the copolyester.

(III b) Preparation of the copolyester P

A reaction mixture of:

1027.2 g of dimethyl terephthalate (5.295 moles)

775 g of dimethyl isophthalate (3.995 moles)

207.2 g of 5-sulphoisophthalic acid dimethyl ester sodium salt (0.7mole)

polycarboxylic acid methyl ester compound CA (0.01 mole)

1240 g of ethylene glycol (20 moles)

220 mg of zinc acetate dihydrate, and

292 mg of antimony(III)oxide

was heated to 160° C. whilst stirring in a nitrogen atmosphere.

At that temperature re-esterification started and methanol wasdistilled. Gradually the temperature was raised to 250° C. over a periodof 3 to 4 h, until no methanol distilled anymore. Thereupon thetemperature was further raised to 255° C. and the reaction mixturesubjected to a reduced pressure of 0.1-0.2 mm Hg. Under these conditionsthe polycondensation took place within a period of about 60 to 100 min.After cooling the solidified copolyester was milled and obtained inpowder form. Depending on the time of polycondensation the intrinsicviscosity of the copolyester was from 0.20 to 0.30 dl/g measured at atemperature of 25° C. in a mixture of phenol and o-dichlorobenzene(60/40 by volume). The Tg value of said copolyester is in the range of67° to 72° C.

The preparation of an aqueous copolyester dispersion is carried out byvigorously stirring the copolyester powder in hot (90°-98° C.) water.Operating that way a copolyester dispersion with average particle sizesmaller than 50 nm and up to 30% by weight can be obtained easily and isused as such

(IV) Preparation of latices A, B, C and D (IV a) Preparation of latex A

An addition copolymer of methylmethacrylate, butadiene and itaconic acidcontaning 47.5% by weight of methylmethacrylate units, 47.5% of byweight of butadiene units and 5% by weight of itaconic acid units isprepared as a latex by classical emulsion polymerisation conducted inaqueous medium in the presence of an initiator compound. A commoninitiator compound is a water-soluble persulphate or a redox systembased e.g. on hydrogen peroxide and a ferrous salt. A survey ofinitiators used in emulsion polymerization is given in High Polymers 9,Emulsion Polymerization, Interscience Publishers, Inc., New York, 1955.

(IV b) Preparation of latex B

An addition copolymer of methylmethacrylate, butadiene and itaconic acidcontaning 67.5% by weight of methylmethacrylate units, 27.5% of byweight of butadiene units and 5% by weight of itaconic acid units isprepared as a latex as described under (IV a).

(IV c) Preparation of latex C

An addition copolymer of vinylidenecholride, methylmethacrylate anditaconic acid contaning 88% by weight of vinylidenechloride units, 10%of by weight of methylmethacrylate units and 2% by weight of itaconicacid units is prepared as a latex as described under (IV a).

(IV d) Preparation of latex D

A mixture of 36 ml of 10% aqueous solution of HOSTAPAL B [tradename ofHoechst Aktiengesellschaft, Frankfurt, Germany for nonyl-phenyl(oxyethylene)₅ --O--SO₃ Na] and 1314 ml of water was heated to 85° C.while nitrogen gas was bubbled through. 77 ml of methyl methacrylate and22.5 ml of a 2% aqueous solution of K₂ S₂ O₈ were added and the reactionmixture was stirred for 30 minutes. Thereupon an additional amount of306 ml of methyl methacrylate and 45 ml of 2% aqueous solution of K₂ S₂O₈ were added simultaneously and dropwise over a period of 30 minutes.Then stirring of the reaction mixture was continued for 2 h at 85° C.The solids content of the latex was 20.2% and the average particle sizeof the dispersed polymethyl methacrylate particles was 88.8 nm. Theglass transition temperature (Tg) of the obtained polymethylmethacrylate was: 122° C.

EXAMPLES 1 TO 6 Preparation of the Coating Compositions

Using the above defined copolyester latex (P), latices A , B and C andthe poly(3,4-ethylenedioxy-thiophene) dispersion (PT1) of preparation(IIa) in a aqueous solution in amounts (in ml) as indicated in followingtabel 1, coating compositions 1 to 6 of examples 1 to 6 were prepared.

                  TABLE 1                                                         ______________________________________                                                   Coating compositions of Examples                                   Ingredient   1      2      3    4    5      6                                 ______________________________________                                        Dispersion PT1                                                                             205    205    205  163  163    199                               Copolyester latex P                                                                        119    119    119  0    0      0                                 Latex A      33     33     0    0    0      0                                 Latex B      0      0      35   0    0      0                                 Latex C      0      0      0    132  119    189                               Surfactant A*                                                                              10     10     10   0    0      0                                 Surfactant B*                                                                              0      0      0    51   51     31.6                              Sorbitol     7.7    7.7    7.7  7.7  7.7    9.4                               N-Methylpyrrolidone                                                                        15     15     15   15   15     0                                 Furancarboxylic                                                                            0      3      0    0    0      0                                 acid                                                                          KIESELSOL 100F*                                                                            0      0      0    0    11     17.5                              Isopropanol  26     26     26   0    0      0                                 Water        574    571    570  620  620    547                               NH.sub.4 OH 25% in water                                                                   0      0      0    0    up to                                                                         pH = 4                                   ______________________________________                                         *Surfactant A: 4.5% aqueous solution of octylfenoxypolyglycolacetic acid      sold by CHEMY company of the Netherlands under the tradename of AKYPOOP-8     *Surfactant B: 9.25 g of a sulfonated linear alkyl chain on basis of          paraffin, sold by the BAYER AG, Germany under the tradename MERSOLAT H76      and 43.5 g of heptadecylbenzimidazole disulfonic acid, disodium salt, sol     by CIBAGEIGY AG, Switserland under the tradename ULTRAVON W mixed in 800      ml water and 200 ml ethanol.                                                  *KIESELSOL 100F: tradename for a colloidal SiO.sub.2 dispersion, particle     size of SiO.sub.2 : 25 to 30 nm, sold by BAYER AG, Germany.              

Coating Procedure

Six test coatings with the compositions 1 to 6 were prepared as follows:The coating compositions as described in table 1 were coated to alongitudinally stretched polyethylene terephtalate film support having athickness of approximately 0.34 nun by air-knife coating. Thecompositons 1 to 5 were applied to said support at a coverage of 60 m²/liter, composition 6 was applied to said support at a coverage of 90 m²/liter. The layer was dried in a hot air stream whereafter the film withcoating compositions 1 to 5 was stretched transversally to 3.3 times itsoriginal width and the film with coating composition 6 was stretched to3.5 times its original width at a temperature of about 87° C. in atenter frame. The final thickness of the film was about 0.100 mm. Thefilm was then conducted into an extension of the tenter frame, where itwas heat-set while being kept under tension at a temperature of 190° C.for about 20 sec. After heat-setting the coated film was cooled andwound up in the normal manner.

The conductivity of the 6 samples was evaluated according to test testprocedures A and B described earlier. The results in table 2 indicatethat for each composition the lateral resistivity is much lower than10¹⁰ Ω/square and that τ≦0.002 msec.

                  TABLE 2                                                         ______________________________________                                               Samples of Example                                                     Property 1       2       3     4     5     6                                  ______________________________________                                        viscosity 22°.sup.+                                                             2.9     2.2     2.9   1.6   1.7   n.a..sup.!                         Coating  60      60      60    60    60    90                                 Thickness                                                                     m.sup.2 /l                                                                    Stretching                                                                             3.3     3.3     3.3   3.3   3.3   3.5                                factor                                                                        Lateral  0.010   0.003   0.020 0.16  0.002 n.a..sup.!                         resistance*                                                                   τ (msec)                                                                           0.002   <.002   <.002 <.002 <.002 <0.002                             ______________________________________                                         *at 30% R.H. expressed as 10.sup.10 (Ω/square)                          .sup.! n.a.: not available,                                                   .sup.+ in cp                                                             

EXAMPLES 7 TO 12

The anstistatic layers of the materials of examples 1 to 5 wereovercoated with a second subbing layer with coating thickness of 30 m²/l to give samples 7 to 11. The coating solution of the second subbinglayer was composed as follows:

Gelatine 11.4 g/1

KIESELSOL 300 F (Tradename of BAYER AG, Germany for a colloidaldispersion of SiO₂) 28.4 g/l

1,2,6 Hexanetriol 0.75 g/l

anionic wetting agent 0.6 g/l

To the above composition was added 0.15 ml of a 5.71% aqueous dispersionof a matting agent MA in the form of crosslinked polymethyl methacrylateparticles having an average particle size of 1.3 μm (ref. publishedEuropean patent application No. 0 466 982).

The anstistatic layer of the material of sample 6 was overcoated with anaqueous solution of latex D to give sample 12.

The aqueous solution of latex D consisted of:

    ______________________________________                                        KELZAN S*              0.3    g                                               NH.sub.4 OH 25% in water                                                                             0.2    ml                                              Latex D                31.1   ml                                              ULTRAVON W*            10     ml                                              N-Methyl-Pyrrolidone   24     ml                                              Water to make          1000   ml                                              ______________________________________                                         *KELZAN is a registered trademark of Merck & Co., Kelco Division, U.S.A.      KELZAN S is according to Technical Bulletin DB19 of Kelco Division            "xanthan gum" which is a polysaccharide containing mannose, glucose and       glucuronic acid repeating units (as a mixed potassium, sodium and calcium     salt).                                                                        *ULTRAVON W is a tradename for the disodium salt of                           2heptadecyl-benzimidazole disulphonic acid sold by CIBA A.G. Switserland)

To the above composition was added 0.15 ml of a 5.71% aqueous dispersionof a matting agent in the form of crosslinked polymethyl metacrylateparticles having an average particle size of 1.3 μm (ref. publishedEuropean patent application No. 0 466 982).

                  TABLE 3                                                         ______________________________________                                               Samples of Example                                                     Property 7       8       9     10    11    12                                 ______________________________________                                        Lateral  0.070   0.900   0.080 0.02  0.03  0.06                               resistance*                                                                   τ (msec)                                                                           <.002   0.720   0.006 <.002 <.002 0.016                              ______________________________________                                         *at 30% R.H. expressed in 10.sup.10 Ω/square                       

EXAMPLES 13 TO 17

The subbed samples 7 to 11 were coated with a gelatinous backing layerto give samples 13 to 17. The samples were aged for 36 hours at 57° C.and 34% R.H., then 24 hours conditionned at 30% R.H. and 21° C. Afterthis procedure the relative resistance and T were measured again, beforeprocessing (BP) and after processing (AP). Also the adhesion of the wetlayer was measured and adhesion tests were performed before and afterprocessing.

In a dry adhesion test the coated layer assemblage was cut reaching thefilm base by means of a razor blade cutting cross-wise under an angle of45° . An adhesive tape (TESAPACK 4122 -tradename of BEIERSDORF AG,Hamburg, Germany) was pressed on the cross-cut areas and torn offabruptly under an angle of 45° . This test is termed "tape-test". Thequality of the dry adhesion was evaluated by giving the peeling off arating from 0 to 4, wherein 0 stands for non-peeling and 4 for completeremoval of the scratched emulsion layer areas.

In a wet adhesion test the coated layer assemblage was dipped for 10minutes into water of 36° C., whereupon the layer assemblage wasscratched cross-wise with a pen tip reaching the film base. The adhesionin wet state was checked by rubbing the wetted layer assemblage withfinger tip for 10 seconds.

The quality of the wet adhesion was evaluated by giving the result ofthe rubbing a rating from 0 to 4, wherein 0 stands for non-removal and 4for complete removal by said rubbing.

The results are given in table 4

                  TABLE 4                                                         ______________________________________                                                   Samples of Examples                                                Property     13      14      15    16    17                                   ______________________________________                                        Lateral resistance*                                                                        0.35    0.32    0.31  0.32  0.33                                 BP                                                                            Lateral resistance**                                                                       0.20    0.30    0.21  0.58  0.17                                 BP                                                                            Lateral resistance*                                                                        0.52    0.41    0.26  0.29  0.29                                 AP                                                                            Lateral resistance**                                                                       n.a.    n.a.    n.a.  n.a.  0.28                                 AP                                                                            τ BP, 30% RH (msec)                                                                    <.022   0.009   0.006 <.002 <.002                                τ AP, 60% RH (msec)                                                                    0.002   <.002   <.002 <.002 <.002                                Adhesion (Tape test)                                                                       0       0       0     0     0                                    BP                                                                            Wet adhesion 0       0       0     0     0                                    Adhesion (Tape test)                                                                       0/1.sup.+                                                                             0       0/1.sup.+                                                                           0/1.sup.+                                                                           0                                    AP                                                                            ______________________________________                                         *at 30% R.H. expressed in 10.sup.10 Ω/square                            **at 60% R.H. expressed in 10.sup.10 Ω/square                           .sup.+ delamination of the PETsupport.                                   

EXAMPLE 18

A polyester support was prepared as described in example 1, except forthe fact that antistatic composition 7 (amounts in ml) was used to givesample 18:

Antistatic coating composition 7:

    ______________________________________                                        Dispersion PT2           180.4                                                Latex C                  189                                                  Surfactant B*            30                                                   Sorbitol                 10.2                                                 KIESELSOL 100F*          17.5                                                 Water ml                 564                                                  Viscosity 22° C. (cp)                                                                           1.8                                                  Coating thickness (m.sup.2 /l)                                                                         90                                                   Stretching factor        3.5                                                  ______________________________________                                         *Surfactant B: 9.25 g of a sulfonated linear alkyl chain on basis of          parafin, sold by BAYER AG, Germany under the tradename MERSOLAT H76 and       43.5 g of heptadecylbenzimidazole disulfonic acid, disodium salt, sold by     CIBAGEIGY AG, Switserland under the tradename ULTRAVON W mixed in 800 ml      water and 200 ml methanol.                                                    *KIESELSOL 100F: tradename for a colloidal SiO.sub.2 dispersion, particle     size of SiO.sub.2 : 25 to 30 nm, sold by the BAYER AG, Germany.          

This antistatic primer layer was coated on both sides of the polyestersupport. The lateral resistance of the material (sample 18) at 30% RHwas 0.0003×10¹⁰ Ω/square and τ was <0.002 msec.

Sample 18 was overcoated on both sides with a second subbing layer asdescribed in example 7, to give sample 19. The lateral resistance of thematerial (sample 19) at 30% R.H. was 0.12 x 10¹⁰ Ω/square and τ was<0.002 msec.

Sample 19 was overcoated on both sides with a gelatin-silverbromide-iodide emulsion [AgBr/AgI 98/2 mole %] was coated at a coverageof silver halide equivalent with 4.00 g of silver nitrate per m2 on eachside of the support. The gelatin to silver halide ratio was 0.4, thesilver halide being expressed as an equivalent amount of silver nitrate.The average grain size of the silver halide was 0.95 μm. This resultedin sample 20. Again the lateral resistance and τ were measured beforeprocessing (BP) and after processing (AP). The results of themeasurements on sample 20 are summarized in table 5.

COMPARATIVE EXAMPLE 1

The emulsion that was coated on sample 19 was coated on both sides of apolyester support that was subbed at both sides with a subbing layeraccording to U.S. Pat. No. 4,123,278, this gave sample 21. Again thelateral resistance and τ were measured before processing (BP) and afterprocessing (AP). The results of the measurements on sample 21 aresummarized in table 5.

                  TABLE 5                                                         ______________________________________                                                           Sample N°                                           Property             20      21                                               ______________________________________                                        Lateral resistance BP at 30%                                                                       0.22    2400                                             R.H. 10.sup.10 Ω/square                                                 Lateral resistance BP at 50%                                                                       0.17     110                                             R.H. 10.sup.10 Ω/square                                                 τ BP at 30% R.H. in msec                                                                       <0.002  4400                                             τ BP at 50% R.H. in msec                                                                       0.009    530                                             ______________________________________                                    

Herefrom it is clear that the lateral resistance and the dischargeduration τ of the photographic material having a anstistatic layeraccording to the present invention (sample 20) are much lower than forthe comparative test subbing layer (sample 20). The antistaticproperties of the samples according to the present invention arepractically independent of relative humidity.

We claim:
 1. A method of preparing a silver halide photographic materialcomprising the steps of:(i) stretching a polyester sheet or web first inone direction and second in a direction perpendicular thereto (ii)coating said hydrophobic polyester sheet or web, either beforestretching or between said first and second stretching operation, on oneor both sides, with a transparent antistatic primer layer, wherein thecoating composition of said transparent antistatic primer layercomprises (1) a dispersion of a polythiophene with conjugated polymerbackbone and a polymeric polyanion compound and (2) a latex polymerhaving hydrophilic functionality (iii) coating one or more hydrophiliccolloid layer(s), including at least one silver halide emulsion layer,on one or both sides of said hydrophobic polyester sheet or web.
 2. Amethod according to claim 1, wherein at least one of said hydrophiliccolloid layers is a subbing layer containing gelatine.
 3. A methodaccording to claim 1, wherein at least one of said hydrophilic colloidlayers is an antihalation layer.
 4. A method according to claim 1,wherein on one side of said hydrophobic polyester sheet or web at leastone silver halide emulsion layer is coated and on the opposite side atleast one non silver halide hydrophilic colloid layer is coated.
 5. Amethod according to claim 1, wherein on both sides of said hydrophobicpolyester sheet or web at least one silver halide emulsion layer iscoated.
 6. A method according to claim 1, wherein said polythiophene hasthiophene nuclei substituted with at least one alkoxy group, or --O(CH₂CH₂)_(n) CH₃ group, n being 1 to 4, or thiophene nuclei whereof twoadjacent carbon atoms of the thiophene ring together with anoxy-alkylene-oxy group form an adjacent ring.
 7. A method according toclaim 1, wherein said polythiophene has structural units correspondingto the following general formula (I): ##STR3## in which: each of R¹ andR² independently represents hydrogen or a C₁₋₄ alkyl group or togetherrepresent an optionally substituted C₁₋₄ alkylene group or acycloalkylene group.
 8. A method according to claim 1, wherein saidpolythiophene has been prepared in the presence of said polymericpolyanion compound by oxidative polymerization of 3,4-dialkoxythiophenesor 3,4-alkylenedioxythiophenes according to the following generalformula (II): ##STR4## in which: R¹ and R² are as defined in claim
 7. 9.A method according to claim 1, wherein said polymeric polyanion compoundis a polymeric carboxylic acid or polymeric sulfonic acid or such acidin salt form.
 10. A method according to claim 9, wherein said polymericpolyanion compound is a polymer comprising styrene sulfonic acid units.11. A method according to claim 1, wherein said latex polymer havinghydrophilic functionality is a copolyester that contains sulfonic acidsalt groups as hydrophilic functionality.
 12. A method according toclaim 11, wherein said latex polymer is a copolyester containingrecurring ester groups derived from ethylene glycol and an acid mixturecontaining (i) terephthalic acid, (ii) isophthalic acid, (iii)5-sulphoisophthalic acid whose sulpho group is in salt form and (iv) apolyfunctional acid producing crosslinks.
 13. A method according toclaim 12, wherein said acid mixture containing terephthalic acid,isophthalic acid and 5-sulphoisophthalic acid whose sulpho group is insalt form and a polyfunctional acid producing crosslinks consistsessentially of from 20 to 60 mole % of isophthalic acid, 6 to 10 mole %of said sulphoisophthalic acid, 0.05 to 1 mole % of cross-linking agentbeing an aromatic polycarboxylic acid compound having at least threecarboxylic acid groups or corresponding acid generating anhydride orester groups, the remainder in said acid mixture being terephthalicacid.
 14. A method according to claim 11, wherein said latex polymerhaving hydrophilic functionality is an addition (co)polymer havinghydrophilic functionality.
 15. A method according to claim 14, whereinsaid addition copolymer is a copolymer formed from 45 to 70 % by weightof a lower (C₁ -C₄) alkyl methacrylate, 25 to 50 % by weight ofbutadiene and from 2 to 5% by weight of an ethylenically unsaturatedcarboxylic acid.
 16. A method according to claim 14, wherein saidaddition copolymer is a copolymer formed from 60 to 90 % by weight ofvinylidenechloride, 9 to 30% of a lower (C1-C4) alkyl methacrylate andfrom 1 to 5% by weight of an ethylenically unsaturated carboxylic acid.